The present invention relates generally to plasma processors including radio frequency (r.f.) responsive coils for exciting gases in vacuum chambers to plasmas that process workpieces in the chamber and more particularly to such a processor including a substantially flat coil having substantially symmetric and asymmetric turns, and to such a coil. Another aspect of the invention relates to a method of plasma processing circular workpieces having different diameters and, more particularly, to a method wherein the same chamber or chambers having the same geometry are coupled to r.f. excitation coils having differing peripheral dimensions when workpieces having differing peripheral lengths are being processed.
One type of processor for treating workpieces with an r.f. plasma in a vacuum chamber includes a coil responsive to an r.f. source. The coil responds to the r.f. source to produce magnetic and electric fields that excite ionizable gas in the chamber to a plasma. Usually the coil is on or adjacent to a dielectric window that extends in a direction generally parallel to a planar horizontally extending surface of the processed workpiece. The excited plasma interacts with the workpiece in the chamber to etch the workpiece or to deposit material on it. The workpiece is typically a semiconductor wafer having a planar circular surface or a solid dielectric plate, e.g., a rectangular glass substrate used in flat panel displays, or a metal plate.
Ogle, U.S. Pat. No. 4,948,458, discloses a multi-turn spiral coil for achieving the above results. The spiral, which is generally of the Archimedes type, extends radially and circumferentially between its interior and exterior terminals connected to the r.f. source via an impedance matching network. Coils of this general type produce oscillating r.f. fields having magnetic and capacitive field components that propagate through the dielectric window to heat electrons in the gas in a portion of the plasma in the chamber close to the window. The oscillating r.f. fields induce in the plasma currents that heat electrons in the plasma. The spatial distribution of the magnetic field in the plasma portion close to the window is a function of the sum of individual magnetic field components produced by each turn of the coil. The magnetic field component produced by each of the turns is a function of the magnitude of r.f. current in each turn which differs for different turns because of transmission line effects of the coil at the frequency of the r.f. source.
For spiral designs as disclosed by and based on the Ogle ""458 patent, the r.f. currents in the spiral coil are distributed to produce a torroidal shaped magnetic field region in the portion of the plasma close to the window, which is where power is absorbed by the gas to excite the gas to a plasma. At low pressures, in the 1.0 to 10 mTorr range, diffusion of the plasma from the ring shaped region produces plasma density peaks just above the workpiece in central and peripheral portions of the chamber, so the peak densities of the ions and electrons which process the workpiece are in proximity to the workpiece center line and workpiece periphery. At intermediate pressure ranges, in the 10 to 100 mTorr range, gas phase collisions of electrons, ions, and neutrons in the plasma prevent substantial diffusion of the plasma charged particles outside of the torroidal region. As a result, there is a relatively high plasma flux in a ring like region of the workpiece but low plasma fluxes in the center and peripheral workpiece portions.
These differing operating conditions result in substantially large plasma flux (i.e., plasma density) variations between the ring and the volumes inside and outside of the ring, resulting in a substantial standard deviation of the plasma flux incident on the workpiece. A measure of plasma flux incident on the workpiece is etch rate of the workpiece in Angstroms per minute; the standard deviation of etch rate uniformity of an Ogle type coil is typically more than 3.0%. The substantial standard deviation of the plasma flux incident on the workpiece has a tendency to cause non-uniform workpiece processing, i.e, different portions of the workpiece are etched to different extents and/or have different amounts of molecules deposited on them.
Many coils have been designed to improve the uniformity of the plasma. The commonly assigned U.S. Pat. No. 5,759,280, Holland et al., issued Jun. 2, 1998, discloses a coil which, in the commercial embodiment, has a diameter of 12xe2x80x3 and is operated in conjunction with a vacuum chamber having a 14.0 inch inner wall circular diameter. The coil applies magnetic and electric fields to the chamber interior via a quartz window having a 14.7 inch diameter and 0.8 inch uniform thickness. Circular semiconductor wafer workpieces having a 200 mm diameter are positioned on a workpiece holder about 4.7xe2x80x3 below a bottom face of the window so the center of each workpiece is coincident with a center line of the coil.
The coil of the ""280 patent produces considerably smaller plasma flux variations across the workpiece than the coil of the ""458 patent. The standard deviation of etch rate uniformity resulting from the plasma flux produced by the coil of the ""280 patent on a 200 mm wafer in such a chamber operating at 5 milliTorr is about 2.0%, a considerable improvement over the standard deviation of approximately 3.0% for a coil of the ""458 patent operating under the same conditions. The coil of the ""280 patent causes the magnetic field to be such that the plasma density in the center of the workpiece is greater than in an intermediate part of the workpiece, which in turn exceeds the plasma density in the periphery of the workpiece. The plasma density variations in the different portions of the chamber for the coil of the ""280 patent are much smaller than those of the coil of the ""458 patent for the same operating conditions as produce the lower standard deviation.
With the advent of circular semiconductor wafers having 300 mm diameters, it has been proposed that the same vacuum chambers be used for plasma processing of circular semiconductor wafers having 200 mm and 300 mm diameters. FIG. 1 is a drawing of a processor that can be used for processing wafers having both diameters. Processors of the type illustrated in FIG. 1 are such that the same processor can be used at different times for both diameters or processors having chambers with the same geometries can be used for separately processing wafers having 200 mm and 300 mm diameters.
The vacuum plasma workpiece processor of FIG. 1 includes vacuum chamber 10, shaped as a cylinder including grounded metal wall 12 having an interior diameter of 20xe2x80x3, metal base plate 14, and circular top plate structure 18, consisting of a dielectric window structure 19, having the same thickness from its center to its periphery and a diameter exceeding the inner diameter of chamber 10 so the window bears against the top edge of wall 12. Sealing of vacuum chamber 10 is provided by conventional gaskets (not shown). The processor of FIG. 1 is typically used for etching a circular semiconductor wafer (i.e., a substrate) or for depositing molecules on such a wafer.
A suitable ionizable gas that can be excited to a plasma state is supplied to the interior of chamber 10 from a gas source (not shown) via port 20 in window 19. The interior of chamber 10 is maintained in a vacuum condition, at a pressure that can vary in the range of 1-100 milliTorr, by a vacuum pump (not shown), connected to port 22 in base plate 14.
The gas in the chamber is excited by a suitable electric source to provide a plasma having a density that is considerably more uniform than the plasma excited by the coil disclosed in the Ogle ""458 patent. The electric source includes a substantially planar metal coil 24 having a square cross-section and a hollow interior; coil 24 is typically made of square copper tubing. Coil 24 is mounted immediately above window 19 and excited by r.f. power source 26, typically having a fixed frequency of 13.56 MHz and usually having a fixed amplitude envelope. The current in coil 24 generates a large enough magnetic field flux in chamber 10 in proximity to window 19 to excite ionizable gas in the chamber to a plasma"".
Impedance matching network 28, connected between output terminals of r.f. source 26 and excitation terminals of coil 24, couples the output of the r.f source to the coil. Impedance matching network 28 includes variable reactances (not shown) which a controller (not shown) varies in a known manner to achieve impedance matching between source 26 and a load including coil 24 and the plasma load the coil drives.
Circular workpiece 32, which can have a 200 mm or 300 mm diameter, is fixedly mounted in chamber 10 to a surface of circular workpiece holder (i.e., chuck or platen) 30; the surface of chuck 30 carrying workpiece 32 is parallel to the surface of window 19. Chuck 30, typically of the electrostatic type, has one of two differing diameters, depending on the diameter of the workpiece being processed at a particular time in chamber 10. Workpiece 32 is usually electrostatically clamped to the surface of chuck 30 by applying a DC potential of a DC power supply (not shown) to one or more electrodes (not shown) of the chuck.
R.f source 31 supplies an r.f. voltage having a constant amplitude envelope to impedance matching network 33, that includes variable reactances (not shown). Matching network 33 couples the output of source 31 to an electrode of chuck 30. A controller (not shown) controls the variable reactances of matching network 33 to match the impedance of source 31 to the load impedance coupled to the electrode of chuck 30. The load coupled to the electrode is primarily the plasma in chamber 10. As is well known, the r.f. voltage that source 31 applies to the electrode of chuck 30 interacts with charge particles in the plasma to produce a DC bias on workpiece 32.
Surrounding planar coil 24 and extending above top end plate 18 is a metal tube or shield 34 having a square cross section within which the coil sits. Shield 34 decouples electromagnetic fields originating in coil 24 from the surrounding environment. The distance between shield 34 and the peripheral regions of coil 24 is large enough to prevent significant absorption by shield 34 of the magnetic fields generated by the peripheral regions of coil 24.
The diameter of cylindrically shaped chamber 10 relative to the outer diameter of coil 24 is large enough to prevent substantial absorption by chamber walls 12 of the magnetic fields generated by the peripheral regions of the coil. The diameter of dielectric window structure 19 is greater than the inner diameter of chamber 10 to such an extent that the entire upper surface of chamber 10 consists of dielectric window structure 19.
The distance between the treated surface of workpiece 32 and the bottom surface of dielectric window structure 19 is chosen to provide the most uniform plasma flux on the exposed, processed surface of the workpiece. Typically, the distance between the workpiece processed surface and the bottom of the dielectric window is approximately 0.3 to 0.4 times the diameter of chamber 10; the inner diameter of chamber 12 is 20xe2x80x3, the diameter of coil 24 having the prior art shape of the ""280 patent is 13xe2x80x3 for a 200 mm diameter wafer, shield 34 has a length of 23 xc2xdxe2x80x3 on each side, and the distance between the workpiece processed surface and the bottom of the dielectric window is 6.0xe2x80x3.
Planar coil 24 functions as a transmission line to produce a standing wave pattern along the length of the coil. The standing wave pattern results in variations in the magnitude of the r.f. voltages and currents along the length of the coil. The dependence of the magnetic flux generated by the coil on the magnitude of these r.f. currents results in differing amounts of plasma being produced in different portions of chamber 10 beneath different portions of the coil. The transmission line behavior of the r.f. current in planar coil 24 increases the amount of magnetic flux generated by the peripheral coil segments relative to the center coil segments. This result is achieved by exciting coil 24 with r.f. so the regions of maximum r.f. current are on the peripheral coil segments.
As illustrated in FIG. 2, the planar coil 24 with the shape of the ""280 patent includes interior substantially semicircular loops 40, 42 and peripheral substantially circular segments 46 and 48 and an intermediate substantially circular segment 44. Each of loops 40 and 42 forms almost a half turn of coil 24, while each of loops 44, 46 and 48 forms almost a full turn; the fill and half turns are connected in series with each other. All of segments 40, 42, 44, 46 and 48 are coaxial with central coil axis 50, in turn coincident with the center axis of chamber 10 and the center of wafer 32 when the wafer is clamped in place on chuck 30. Opposite excitation terminals 52 and 54, in the center portion of coil 24, are respectively coupled by leads 48 and 56 to opposite terminals of r.f. source 26 via matching network 28 and one electrode of capacitor 80, the other electrode of which is grounded. Terminal 60, at the end of loop 40 opposite from terminal 52, is connected to end terminal 66 of outer loop segment 48 by metal strap 64 which is located in a region somewhat above the plane of coil 24. The spacing between adjacent segments 40, 42, 44, 46 and 48 and the spacing between strap 64 and the remainder of coil 24 are great enough to prevent arcing between them. The radii of the outer edges of segments 40, 42, 44, 46 and 48 are respectively 2xe2x80x3, 2xe2x80x3, 3.5xe2x80x3, 5.5xe2x80x3 and 6.5xe2x80x3.
Segment 48 has a second terminal 68 slightly less than 360xc2x0 from terminal 66; terminal 68 is connected to terminal 70 of loop segment 46 via strap 72. Loop 46, having an angular extent of almost 360xc2x0, has a second end terminal 74 connected to terminal 76 of loop 44 via radially and circumferentially extending strap 78. Loop 44, having an angular extent of almost 360xc2x0, has a second end terminal 80 which is connected by radially and circumferentially extending strap 82 to terminal 62 at the end of segment 42 opposite from terminal 54.
Capacitor 80, having a capacitive impedance Zcap=1/(j2xcfx80fC), where j ={square root over (xe2x88x921)}, f is the frequency of r.f. source 26, and C is the capacitance of capacitor 30, shifts the phase and therefore location of the voltage and current distribution across the entire length of coil 24. The voltage and current distribution are shifted in coil 24 so the coil produces r.f. electric and magnetic fields which provide plasma flux on the processed surface of workpiece 32 that is considerably more uniform than the flux resulting from energization of a coil of the type Ogle discloses in the ""498 patent.
The voltage and current of coil 24 are distributed by selecting the value of capacitor 80 so the peak-to-peak r.f. current at coil terminal 54 is a minimum and equals the peak-to-peak r.f. current at coil terminal 52. At this condition, the coil has opposite polarity maximum peak-to-peak r.f. voltages at terminals 52 and 54 and the coil maximum r.f. current occurs near conductive strap 72. The distribution of r.f. voltages and currents in the coil can be approximated by
Vpkpk(X)=Vopkpk cos [xcex2(x+xo)]
and
Ipkpk(X)=Iopkpk sin [xcex2(x+xo)],
where:
x is the linear distance measured from terminal 54 of the coil,:
xcex2is the angular frequency of r.f. source 26 (i.e. 2xcfx80f), divided by c, the speed of light,
xo is an offset from zero which is determined by the value of the capacitor 80, and
Vopkpk and Iopkpk are respectively the maximum r.f. peak-to-peak voltages and currents in the coil.
The value of capacitor 80 is selected so xo is about 0.15 times the wavelength (xcex=c/f) of the r.f. current flowing in the coil.
The peripheral regions of coil 24 produce greater magnetic flux than the center region of the coil because the magnitude of the peak-to peak r.f. current is greater in the peripheral segment of the coil relative to the magnitude of the peak-to-peak currents of the central segments. The maximum peak-to-peak r.f. current amplitude occurs in substantially circular loop segment 46. The amplitudes of the peak-to-peak r.f. currents in adjacent loop segments 44 and 48 and in loop segment 46 and the spacing of loop segments 44, 46 and 48 from each other are such that magnetic fluxes from these three loop segments are combined in space to provide a total magnetic flux density, just under window 19, having a maximum value over a relatively broad annular area. The annular area extends from between loop segments 46 and 48 to between intermediate segment 44 and interior segments 40 and 42.
The variations in the r.f. current magnitude flowing in different parts of the coil are spatially averaged to assist in causing a more uniform plasma to be incident on wafer 32 than is attained by the coil of the Ogle ""498 patent. It was previously thought that spatially averaging these different current values in the different parts of the coil substantially prevented substantial non-radial asymmetries in the plasma density, particularly at regions of high r.f. current in the coil segments near the coil periphery. The total magnetic flux is also considerably more constant as a function of angular coordinate xcex8 than is the case for the coil of the Ogle patent, (where xcex8 represents the angle about the coil periphery relative to a reference angle that extends through the coil center point 50xc2x0, e.g., the reference angle extends horizontally in FIG. 2 to the left of center point 50).
A spatially averaged magnetic flux which is constant along a particular coordinate value xcex8 provides a plasma which is more radially symmetric along xcex8 than is the case of plasma resulting from the coil disclosed in the Ogle ""458 patent. The amplitudes of the peak-to-peak r.f currents in the two substantially sermicircular, equal radius segments 40 and 42 are significantly less than the amplitudes of the currents in the other segments. Segments 40 and 42 derive sufficient magnetic fluxes which are spatially averaged with the magnetic fluxes derived from the other segments 44, 46 and 48 so the plasma flux generated at the level of the processed surface of workpiece 32 across the diameter of the chamber is considerably more uniform than is achieved with the coil of the Ogle patent.
The electrostatic (i.e., capacitive) coupling to the plasma of the voltages at different portions of planar coil 24 (for example, between portions of loop segments 46 and 48 at the same angular coordinate position xcex8) has an influence on the uniformity of the generated plasma flux. The capacitive coupling of these voltages to the plasma depends on the magnitude of the peak-to-peak voltages occurring in the coil segments, as well as the thickness and dielectric material of window 19 which separates the coil from the plasma. The influence of the capacitive currents produced by the r.f. voltages is minimized by causing the highest r.f. peak-to-peak voltages to occur at terminals 52 and 54. The geometry of coil 24 and proper selection of the value of capacitor 80 cause the highest r.f. peak-to-peak voltages to occur at terminals 52 and 54. R.f. excitation of planar coil 24 produces a substantially planar plasma having a flux that is considerably more uniform completely across workpiece 32 than that resulting from the coil of the Ogle ""458 patent.
As the features of integrated circuits have become smaller and smaller, we have found that the uniformity of the plasma incident on a 200 mm wafer produced by the coil described in connection with FIG. 2 is frequently not sufficient. There is an asymmetric diametric plasma flux distribution on the 200 mm wafers formed with the processor of FIG. 1 when the plasma is excited by the described 13 inch diameter coil. The asymmetry is sufficient to have an adverse effect on semiconductor devices having 0.18 micrometer features formed on a 200 mm semiconductor wafer. In particular, we found that the etch rate on circular 200 mm polysilicon wafer 71 is as shown in FIG. 3 by regions 72, 74, 76, 78 and 80 when the wafer was etched at a vacuum of approximately 20 mTorr in the described processor of FIG. 1 while gas in the processor was excited to a plasma by the coil of FIG. 2 being connected to a 13.56 MHz r.f. source 26. The periphery of wafer 71 includes a positioning notch, shown in FIG. 3 as point 73. A positioning device (not shown) placed wafer 71 in chamber 10 so that the notch was offset about 10xc2x0 clockwise from vertical line 75, FIG. 3, extending through wafer center point 70.
Wafer 71 center point 70 is surrounded by region 72 having a maximum etch rate of 2378 Angstroms per minute. Region 72 is somewhat asymmetrical, having a greater extent to the left of center point 70 than to the right, as illustrated in FIG. 3. Surrounding region 72 is region 74, having an etch rate between 2378 and 2396 Angstroms per minute. Region 74 extends to the left edge of wafer 71 spanning an arc length of approximately 100xc2x0 along the left edge of the wafer. Region 74 also extends somewhat to the right side of region 72 and has an approximately circular peripheral contour within the wafer.
Generally crescent shaped regions 76, 78 and 80, to the right, as well as above and below center point 70 and region 74, respectively represent etch rates in the ranges of 2398-2418 Angstroms per minute, 2418-2438 Angstroms per minute and above 2438 Angstroms per minute. Region 76 has an arc length on the periphery of wafer 71 of about 30xc2x0 above and about 15xc2x0 below center point 70 on the periphery of the wafer; region 78 has an arc length on the periphery of wafer 71 of approximately 15xc2x0 above center point 70 and about 20xc2x0 below center point 70, while region 80 has an arc length of about 170xc2x0 along the right edge of the wafer. In general, there is a monotonic variation of etch rate from the left edge of region 76 to the periphery of region 80 along the edge of the wafer.
Wafer 71 has an average etch rate of 2412 Angstroms per minute and a non-uniformity of 1.4% at a standard deviation of one sigma. This high degree of uniformity was frequently sufficient for processing of prior art 200 mm wafers having features greater than 0.18 micrometers, i.e., to etch materials from such wafers and to deposit materials on the wafers. However, with the advances in integrated circuitry resulting in features as small and smaller than 0.18 microns, the uniformity illustrated in FIG. 3 achieved with the coil of FIG. 2 is not always adequate.
An analysis of FIG. 3 reveals an asymmetry in plasma density associated with regions 76, 78 and 80, all of which are essentially to the right side of center point 70, as well as regions 72 and 74. We have realized that reducing the plasma density variations associated with regions 76, 78 and 80 is very likely to enable the plasma density uniformity to be increased to a greater extent than is achieved with the coil illustrated in FIG. 2.
It is, accordingly, an object of the present invention to provide a new and improved vacuum plasma processor and r.f. plasma excitation coil for achieving highly uniform plasma flux (i.e., plasma density) on a circular workpiece.
An additional object of the invention is to provide a new and improved vacuum plasma processor having an r.f. plasma excited by a coil having a new and improved geometry for achieving greater uniformity of plasma density on a circular workpiece.
A further object of the invention is to provide a new and improved plasma processor and coil capable of etching wafers to a non-uniformity less than 1.4% at a standard deviation of one sigma.
Another object of the invention is to provide a new and improved vacuum plasma processor having a new and improved r.f. excitation coil, particularly adapted for use in manufacturing integrated circuits having features as small and smaller than 0.18 microns.
An additional object of the invention is to provide a new and improved method of operating a plasma processor such that workpieces having the same geometry but differing sizes can be processed in the same processor chamber or in chambers having the same geometry.
An added object of the invention is to provide a new and improved method of operating a plasma processor such that circular semiconductor wafers having 200 and 300 mm diameters can be processed in the same processor chamber or in chambers having the same geometry.
In accordance with one aspect of the present invention, a substantially planar coil for supplying an r.f. plasma excitation field to an ionizable gas of a vacuum plasma processor for a circular workpiece adapted to be placed at a particular location on a workpiece holder includes (a) plural coaxial turns substantially symmetrically arranged with respect to a center point of the coil, and (b) at least one further turn asymmetrically arranged with respect to the coil center point.
The invention also concerns a vacuum plasma processor including such a coil.
Another aspect of the invention relates to a vacuum plasma processor for processing circular workpieces to achieve greater plasma density uniformity on the workpieces than another vacuum plasma processor. The another processor includes a substantially planar r.f. excited coil having four concentric series connected circular turns and two centrally located excitation terminals connected to an r.f. excitation source. The another processor, when excited, causes plasma density distribution on the workpiece to be diametrically asymmetrical such that plasma density along a substantial arc length of the workpiece edge on a first side of the workpiece is substantially different from the plasma density along a substantial arc length of the workpiece edge on a second side of the workpiece. The arc lengths on the first and second sides are approximately diametrically opposed to each other. The processor comprises (a) a vacuum chamber for processing the workpieces with a plasma, (b) a holder in the chamber for the workpieces, (c) a substantially planar coil for exciting gas in the chamber into the plasma, and (d) an r.f. source for energizing the coil. The coil includes (a) plural circular coaxial concentric turns having a common center point, (b) central excitation terminals having connections to be driven by the r.f source, and (c) at least one further turn having a geometry different from the other turns to reduce substantially the diametric asymmetry of the plasma density and provide greater plasma density uniformity on the workpiece than the another plasma processor operating under the same conditions.
Preferably the at least one further turn includes arcuate portions having differing spacings from the center point so (a) the arcuate portion closer to the center point is aligned with the portion of the workpiece having a higher plasma density in the another processor, and (b) the arcuate portion farther from the center point is aligned with the portion of the workpiece having a lower plasma density in the another processor.
The at least one further turn in one embodiment includes first and second arcuate portions. The first and second portions are sectors of circles having different radii and a common center point that is substantially the same as the center point of the plural turns. In all embodiments of the invention, the at least one further turn preferably has a shape that is closed except for connections to adjacent turns.
In another embodiment, the at least one further turn has the shape of a circle having a center displaced from the common center point toward the side of the workpiece having substantially lower plasma density during processing by the another processor.
In one preferred embodiment, the coil has only four turns, three of which are the plural turns, and only one of which is the at least one further turn. The further turn is between the plural turns having the largest and second smallest radii. The excitation terminals are on the turn having the smallest radius. The turns having progressively greater separation from the center point are connected directly to each other, while the turns having the largest and smallest radii are connected directly to each other.
In another embodiment, the at least one further turn includes (a) a first portion that is a sector of a circle having a center that is coincident with the coil center point, (b) a second portion that extends substantially straight from a first end of the first portion and (c) a third portion that extends substantially straight from a second end of the first portion. The first portion is preferably split into first and second segments respectively having third and fourth ends connected to the r.f. source.
Such a coil preferably includes first and second further turns. The first further turn is closer to the coil center point than the second of the further turns. Both of the at least one further turns have the previously discussed shape. The sectors of the circles of both of the further turns are arcuately aligned. The straight portions of both of the further turns extend substantially parallel to each other. The first and second of the further turns are the turns of the coil closest to the coil center point.
A further aspect of the invention relates to a method of processing workpieces having differing first and second peripheral lengths and geometrically similar shapes in the same vacuum plasma processing chamber or vacuum plasma processing chambers having the same geometries. The method includes the step of causing r.f. plasma excitation coils having first and second different peripheral dimensions to be coupled to ionizable gas in the chamber or chambers while the workpieces having the first and second peripheral lengths are being respectively processed in the chamber or chambers while the chamber or chambers are in vacuo and r.f. energy is supplied to the coils to excite the gas to a processing plasma. The first peripheral length exceeds the second peripheral length, and the first peripheral dimension is greater than the second peripheral dimension.
Preferably, the workpieces are circular and each of the coils includes plural substantially circular coaxial turns having a center point aligned with center points of the circular workpieces and each of the coils includes at least one turn that is asymmetric with respect to the coil center point.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed descriptions of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.